Some gods chain Prometheus to a rock. This isn't starting out well for our intrepid hero.
Prometheus complains for a while, and then the Chorus shows up to feel sorry for him.
It's okay: Zeus won't be able to lord it over him forever, because Prometheus knows the future.
At the urging of the Chorus, Prometheus tells a long story about the war between the Titans (the old gods) and the Olympians (the new gods).
It wraps up with Prometheus explaining that he gave mortals medicine and technology, and mining, and a little thing called "hope"—and, oh yeah, fire.
This blows the Chorus's mind, and the agree to listen to Prometheus prophesy.
Hey-o! Here comes Oceanus, the Chorus's dad. He volunteers to help out Prometheus, but Prometheus rejects the offer since it would mean sucking up to Zeus.
The Chorus sings and dances for a while, and then chats with Prometheus about miserable humanity some more.
When Io in cow-form charges on stage, Prometheus volunteers to tell her future. It's pretty grim, but not as bad as Prometheus's.
Also, one of Io's descendants is going to be the guy to free Prometheus. Go, Io!
And then, another visitor appears: Hermes, who's come to get Prometheus to reveal Zeus's eventual downfall. He's obviously unsuccessful. After a little earth-shaking, lighting-bolting, and rock-splitting, the ground opens up and Prometheus tumbles into the earth.